Materials with a high spin polarization are required to achieve high performance spintronics devices, such as magnetic random access memory (MRAM), spin metal-oxide-semiconductor field effect transistor (spin MOSFET), tunnel magnetoresistance (TMR) used for a read head of a hard disk drive, giant magnetoresistance (GMR), spin torque oscillator (STO), and nonlocal spin valve (NLSV) which has been gained attention as a next generation read head. Co-based Heusler alloys are the candidates for highly spin polarized material, because some of the Co-based Heusler alloys are predicted to be a half-metal (half-metal: no density of states in one band at Fermi level, 100% spin polarization) and have a Curie temperature sufficiently higher than a room temperature.
Heusler alloys have an L21 ordered structure and their chemical formula is X2YZ. However, since the kinetics of L21 ordering is weak, a perfect L21 structure may not be obtained but disorder structures are easily formed. The structure with the disordering between Y and Z is a B2 structure. That between X, Y and Z is an A2 structure. The reduction of the spin polarization by the disordering is theoretically showed. Experimentally, Co2MnSi which is predicted to be a half-metal has the spin polarization of 0.59 (59%) estimated by a point contact Andreev reflection (PCAR) method. The low spin polarization is due to the disorder structure.
The search for high spin polarized materials have been carried out up to now. According to Non Patent Literature 1, the high spin polarization of 75% was obtained by the substitution of Ga for Ge in Co2MnGe. Since the Co2MnGaGe is an intermetallic compound, high degree of L21 order can be achieved. However, Mn in Co2MnGaGe readily causes an oxidation and diffuses into an Ag layer used for a non-magnetic metal of current-perpendicular-to-plane (CPP)-GMR devices. On the other hand, Co2Fe-based Heusler alloys do not have such problems. Therefore, the development of Co2Fe-based Heusler alloys with high spin polarization (more than 0.65 by PCAR measurement) is expected.
Among the Co2Fe-based Heusler alloys, Co2FeAl0.5Si0.5, which is formed by the substation of Al with Si in Co2FeSi, is well known as a highly spin polarized material. In Non Patent Literature 3, it has the spin polarization of 0.6 (60%). CPP-GMR using this alloy has large magnetoresistance change of 34% at RT and resistance-area product of 8 mΩμm2. The spin polarization of Co2FeAl0.5Si0.5 was estimated to be 0.7 (70%) and 0.77 (77%) at RT and 14 K, respectively. In order to improve the properties of spintronics devices, the development of the highly spin polarized materials higher than Co2FeAlSi is strongly required.